The deficiency in the industry has been the inability to balance the necessary hydrophilicity for emulsion polymerization with ultimate resistance to water in the resulting polymer products. Additionally, it has been difficult and costly to incorporate active functionality into emulsion polymers due to copolymerization considerations as well as to a lack of chemical versatility in the available functional monomers. Functional monomers are often not stable to the conditions of polymerization in aqueous emulsion systems and are, therefore, unsuited for providing appropriate and desired physical and mechanical property development in the ultimate uses of the emulsion polymer products. Typically, the functionality utilized to crosslink these emulsion polymers has been incorporated through copolymerization with the polymer itself.
Normally, aqueous polymer emulsions contain either hydrophilic polymers such as poly(vinyl alcohol) [PVOH], poly(ethylene oxide), or poly(vinyl pyrrolidone) or surfactants such as alkylphenol ethoxylates, alkyl sulfonates, or alkyl ammonium salts to allow stable polymerization in an aqueous continuous phase. Due to the necessary hydrophilicity of these materials, emulsion polymers prepared containing these materials remain sensitive to the presence of water even after polymers are cast into a solid film, as would be utilized for adhesives, for example. This water sensitivity leads to limitations in the types of applications where these emulsion polymers can be used. Emulsion polymers exhibiting this water sensitivity cannot typically be used, for example, in outdoor applications where exposure to the elements would cause degradation of polymer performance and ultimately in failure of the polymer product.
Previous solutions to this deficiency have involved incorporation of functional monomers into the emulsion polymers themselves where they remain until catalyzed or promoted to reaction by some mechanism. Typical monomers of this type are N-methylolacrylamide, hydroxyethyl acrylate, glycidyl (meth)acrylate and similar monomers. Several technical problems limit the utility of these types of monomers in addition to economic considerations. Such hydrophilic monomers are soluble in water and are therefore difficult to incorporate effectively into emulsion polymer compositions. Polymerization of these monomers in the aqueous phase can result in serious difficulty in controlling the viscosity of the dispersion and can lead to inadequate performance as crosslinking agents. Because the functional monomers are usually incorporated into the polymer with dissimilar monomers, consideration must be given for the differing reactivities of the two or more types of monomers in the system. These reactivity considerations can affect the available compositions for the individual monomers in the overall product as well as the distribution of the dissimilar monomers within the polymer molecules, thus limiting their effectiveness.
Crosslinking monomers are desirably incorporated into the polymer formed during the emulsion polymerization process. These active functionalities are then catalyzed or promoted to reaction either by a chemical process such as addition of an acid catalyst or by a physical process such as the application of heat or by some combination of both. Inherent to such utilization of these crosslinking monomers is their incorporation into the polymer formed during the polymerization in the system. Later, through catalysis or the promoted reaction, these functionalities react to form a larger molecular network which helps improve many of the physical and mechanical properties of the emulsion polymer such as the tensile strength and modulus and helps resist the degradation of properties on exposure to water or solvents.
Emulsion polymerization of monomers such as vinyl acetate, vinyl chloride and vinylidene chloride and their copolymers with ethylene or acrylates has long been known to be stabilized by protective colloids, water soluble polymers such as PVOH and hydroxyethyl cellulose. However, it has been difficult to polymerize exclusively acrylic monomers in the presence of the water soluble polymers at useful levels above 0.5% based on polymer solids.
Previous attempts to incorporate water soluble polymers such as PVOH into acrylic emulsion polymer products have focused on the post addition of the PVOH to the already polymerized acrylic emulsion. In U.S. Pat. No. 4,280,942, for example, the PVOH is added after the monomer has been completely added to the emulsion reaction. While this patent teaches the use of PVOH in an acrylic contact adhesive formulation, it does not specify in Example 1 the method of polymerization nor the means of PVOH addition to the emulsion product. Example 5 indicates that the PVOH solution is added to the emulsion after all of the monomer has been added. In addition, the examples disclosed in this patent also rely on the use of additional surfactants which act as primary emulsifier for the emulsion polymerization.
Other attempts (U.S. Pat. No. 5,011,883) have utilized copolymers of vinyl monomers such as vinyl acetate with acrylates to achieve incorporation of the PVOH into the emulsion product. This approach also has limitations in the level of vinyl acetate which can be homogeneously incorporated into the emulsion product and in the compromise in properties such as Tg that are expected due to copolymerization of the vinyl monomer with the acrylic monomers.
Other efforts have shown that incorporation of amine functionality into a polymer latex composition of acrylic polymers is desirable but the routes used to achieve this incorporation are difficult. EP 072,684 discloses the quaternization of amine functional monomers or polymers with epichlorohydrin as a route to incorporation of this functionality for adhesive compositions. EP 0,489,524 describes the preparation of amine containing emulsion products which incorporate amine functionality through the use of ammonium salts of fatty acids which are subsequently decomposed at elevated temperatures to produce acid functionality and ammonia. In that case, the conversion of the ammonium functionality used for stabilization to the thermalized acid fore results in the transformation of the compound from the water soluble to a water insoluble species.
Cationic PVOH has been used for the emulsion polymerization of acrylic and vinyl monomers (U.S. Pat. No. 4,308,189 and J 60058403A). These methods of cationic incorporation into the emulsion product rely on the copolymerization of (meth)acrylamide type monomers which typically limit the compositions of the stabilizing copolymers due to reactivity differences with vinyl acetate.